a) Technical Field
The present invention relates to a method of analyzing the astigmatic difference of an optical device such as an optical pickup device or the like. It particularly relates to a method of analyzing the astigmatic difference inherent to an optical device while correcting the astigmatic difference inherent to a condensation performance analysis system such as a spot analysis system or the like.
b) Background Technology
Recently, optical pickup devices have often been used to write data onto optical disks such as CDs and MDs and these optical disks are expected to store a large volume of data. To meet the trend of large data storage capacity, a spot size for recording the data must be minimized in order to increase the density of data recording. A reduction in wavelength plays an important role in accomplishing this. Nevertheless, a laser beam has an astigmatism characteristic that causes a person to see emitted points at different positions spread apart when viewed in the vertical and the horizontal directions or the laser beam appears substantially elliptical in cross section. As a result, it is a general practice that an optical pickup device is designed to have an optical system for correcting the astigmatic difference. The amount by which the astigmatic difference needs to be corrected is measured by analyzing the optical pickup device optically.
As described above, the condensing performance of an optical pickup device is usually determined through spot analysis. The indices of the spot analysis are each of the aberrations represented by the spot diameter, focal depth, spherical aberration, coma aberration, and astigmatism, and the spot analysis is performed by a spot analysis system. For example, known optical pickup analysis systems include the OPT-W series manufactured by Nissho Electronics Corporation, which analyzes the spot condensed by an optical pickup device used for the optical system of a microscope.
The basic configuration of the spot analysis system is the same as that of a microscope in that it uses the optical system used for a microscope. For this reason, the spot analysis system itself has aberrations such as a spherical aberration, coma aberration, and astigmatism that prevent the spot analysis system from accurately analyzing astigmatic difference.
Therefore, to analyze a spot accurately for the optical pickup device by utilizing a spot analysis system, aberrations that are present in the spot analysis system must be corrected. For such aberrations, a spherical aberration can be corrected by changing the thickness of a cover glass in the spot analysis system, and a coma aberration can be corrected by tiling the cover glass.